1. Field of the Invention
The present invention relates to an igniter transformer and, in particular, to a coil structure of the igniter transformer.
2. Description of the Related Art
Conventionally, igniter transformers have been used as high-voltage generation units to ignite regular HID lamps (High Intensity Discharge Lamps), which are typically used for car headlights. An igniter transformer, as is shown schematically in FIG. 10, includes a magnetic core 21 having a substantially elliptic cross-section, a secondary coil 22 surrounding the magnetic core 21, and a primary coil 23 further surrounding the secondary coil 22, as is disclosed in Japanese Unexamined Patent Application Publication No. 2002-93635.
The secondary coil 22 is composed of a flat copper wire which is edgewise wound, that is to say, it is wound such that the larger surfaces of the flat wire face each other in the turns. The magnetic core 21 is disposed in the center hole of the secondary coil 22 with or without an insulating film (not shown in the drawing) being disposed therebetween. Similarly, the primary coil 23 is composed of a flat wire which is spirally wound around an insulating bobbin 24 that covers the secondary coil 22 in a so-called ribbon winding manner where one of the larger surfaces of the wire is in contact with the outer surface of the insulating bobbin 24.
This conventional igniter transformer has the following disadvantages. In order to make the igniter transformer thinner or lower in profile, it is necessary to flatten the secondary coil 22 so that the secondary coil 22 follows the cross-sectional shape of the magnetic core 21. However, the flat wire forming the edgewise-wound secondary coil 22 has high tensile strength and is thus difficult to process.
As shown in FIG. 11, which illustrates an enlarged side view of a bent portion 25, if the secondary coil 22 is flattened, partial bending of the flat wire will compress an inside portion 25a of the bent portion 25 more strongly than an outside portion 25b. Such partial bending may cause wrinkles 26 in the inside portion 25a of the bent portion 25, or may reduce the thickness of the outside portion 25b while increasing the thickness of the inside portion 25a, as shown in FIG. 12, which illustrates an enlarged sectional view of the relevant portion.
It is difficult to achieve a radius of curvature R that is less than 7.7 millimeters in the case of a flat wire having a width W of 1.5 millimeters and a thickness T of 75 micrometers. The widened inside portion 25a of the bent portion 25 increases the entire length of the edgewise-wound secondary coil 22 along the axis X of the secondary coil 22. As a result, the space factor of the secondary coil 22 is reduced from about 90%, which is normal, to about 70%.
Since the flat wire forming the secondary coil 22 has a rectangular cross-section, it is difficult to form an insulating coating (not shown) having a uniform thickness over the entire surface of the flat wire without a special electrodeposition process. The flat wire of the secondary coil 22 requires an insulating coating having a sufficient thickness of, for example, about 40 micrometers to maintain a desired withstand voltage. Thus, the space factor of the secondary coil 22 is reduced. Furthermore, a flat wire inherently causes eddy current loss, which may reduce the voltage generated by the igniter transformer.
Generating a high voltage requires close coupling of the secondary coil 22 and the primary coil 23 in a conventional igniter transformer. When the primary coil 23 is wound using the flat wire in the ribbon winding manner (shown in FIG. 13 illustrating a plan view of the winding) and a high output voltage, for example, 25 kV is required, a high-voltage terminal 23a of the primary coil 23 must be disposed substantially at the center of the entire length of the secondary coil 22 along the axis X of the secondary coil 22, namely, at the high-voltage side of the secondary coil 22 far beyond the maximum coupling point. Consequently, the inter-coil withstand voltage characteristics of the igniter transformer are degraded.